Method and material for heat treating fusible material



5"; 21, 1963 H. F. STERLING 3,090,673

METHOD AND MATERIAL FOR HEAT TREATING FUSIBLE MATERIAL Filed May 1, 1959 [IGOR/1.6. SOURCE 2o 15.6.02 AC.

QSOURCE Inventor Y H .F STERL NG- Attorney Patented May 21, 1963 plies to all arrangements incorporating the present inven- 3,090,673 'tion. A crystal of silicon may be pulled from the molten METHOD AND MATERIAL FOR HEAT TREATING silicon 4 by the Czochralski method.

FUSIBLE MATERIAL Henley Frank Sterling, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Filed May 1, 1959, Ser. No. 810,422 Claims priority, application Great Britain June 12, 1958 2 (Ilaims. (Cl. 232.73)

This invention relates to a method and apparatus for heat treating fusible material.

In the co-pending application Serial No. 708,100, filed January 10, 1958 it is proposed to process fusible material, for example semiconductor material such as silicon,

by melting it in a hollow-walled crucible of a metal of 'to process material without introducing any contamination from the crucible in which the processing is carried out.

I have now established that the freedom from contamination of the processed material is due to the fact that the molten materials do not Wet the surface of the cmcible made of a metal of high electrical and thermal conductivity such as silver. According to the present invention the melting of the material to be processed in such a crucible is carried out by passing a direct current or a low frequency alternating current directly through the material in such a crucible. This method has the advantage that it is much more efficient in power consumption.

Specific embodiments of apparatus according to the to the invention will now be described in detail with reference to the accompanying drawings, which are diagrammatic only and in which:

FIG. 1 shows an arrangement for melting material such as silicon which is in the form of small lumps or powder in order to consolidate it into a coherent rod,

FIG. 2. shows a zone-refining system incorporating the principle of the present invention, and

FIG. 3 shows another zone refining system embodying the principle of the present invention.

Referring first to FIG. 1, there is shown a silver crucible 1, through which cooling water is circulated by pipes 2 and 3. Silicon 4 in the form of lumps or powder is placed in the crucible 1. A silver finger 5, which has the form of a test tube 6 made of silver and having Walls of the order of /2 mm. thick and which has cooling water passed through it by pipes 7 and 8, has its lower end within the silicon 4 as shown.

The silver crucible 1 and the silver finger 6 are connected to opposite sides of a source 9 of electric current so that they act as electrodes for passing current directly through the silicon 4. The source 9 of heating current may be a D.C. source or a low frequency A.C. source. In the latter case a frequency of the order of 50 c./s. is preferred although frequencies up to 10 kc./s. may be used.

When the current is switched on and the silicon 4 is in the solid state, a high voltage is necessary to pass a current through the silicon. As the silicon 4 warms up and melts, so its resistance decreases and the voltage applied from the source 9 has to be decreased. This ap- In FIG. 2. there is shown a silver crucible 10, through which cooling water is circulated by pipes 11 and 12. When a zone refining process is being carried out in the crucible 10 there is solid silicon 13 and a liquid zone 14 of silicon around a silver finger 15, which is exactly similar to the silver finger 5 of FIG. 1. Current is passing from a source 19 round the circuit consisting of the crucible 10, the molten zone 14 of the silicon, and the finger 15. The silver finger 15 is arranged to be movable to and fro along the length of the crucible 10 as indicated by the arrows 20, so that the molten zone is moved gradually along during the Zone refining process.

It is possible for more than one silver finger such as 15 to be used, so that a number of molten zones 14 are present during the Zone refining process.

Referring now to FIG. 3 there is shown diagrammatically a portion of a new form of crucible which consists essentially of a series of silver members 21, each made from a silver pipe bent into the form of a leter U and cooled by means of a circulating fluid. FIG. 3 shows only the top surface of the silver members 21 and the manner in which they are arranged side by side in the form of a boat, into which the material to be zone refined is placed. The gap 22 separating each member 21 from its neighbour is wide enough to prevent arcing when the zone refining process described hereinafter is carried out and yet narrow enough for surface tension to prevent spillage of molten material through them. A suitable size for the gaps 22 is of the order of 30-40 'thousandths of an inch, and for each silver member 21 a pipe of the order of A3 of an inch diameter is used.

An electrical terminal 23 is provided for each member 21. In operation a crucible consisting of more members 21 than is actually shown in FIG. 3 has a rod of a material to be refined, such as silicon, placed in it. An electric current is then arranged to flow between a selected group of adjacent members 2 1 by connecting their terminals 23 to a suitable source of D.C. or low frequency A.C. For example, if the group consists of four members the terminals of the first and third are connected to one terminal of the supply voltage and those of the second and fourth to the other. A molten zone will then be formed adjacent to the four members. The molten zone may be transferred along the length of the rod by altering the connections to the members 21, the connection being removed from the first member and placed on the fifth in the example given above.

In this arrangement it is again possible to have more than one molten zone at a time by connecting a number of groups of members 21 to the source of power at the same time.

The arrangement shown in FIG. 3 may be used with an arrangement such as is shown in FIG. 2 i.e. by providing a single electrode like 15 in FIG. 2 to which one terminal of the current source is connected, such electrode being traversed along the crucible comprising the members 21 and gaps 22. The other terminal of the current source is then connected to the terminals of those members 21 which at any one time are adjacent to the electrode finger. In this case also a plurality of electrodes such as 15 may be provided so as to traverse a plurality of zones along the body of silicon. On these may he an electrode such as 15 for each member 21 of the crucible, the source being connected progressively to a terminal 23 and an electrode 15 along the length of the body of silicon.

It is to be noted that in zone refining using a finger electrode such as 15, FIG. 2, it may be necessary to remove the electrode from the body of silicon being zone refined to return it to the other end of the body in order to execute another pass in the same direction. It is important to disconnect the current source from such electrode before removing it from contact with the silicon.

posed on an alternating current may help ionised impurities to be moved to one end of the body being treated, thus assisting the process of zone refining.

*It Will be appreciated that, although the crucible in each of theexamples given has been described as being made of silver, any other metal of high electrical and thermal conductivity, 'e.g., copper'o'r gold, may be used. Again the apparatus of any of the examples may be used for processing a wide range of materials including, for example silicon, germanium, titanium and nickel.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by Way of example and not as a limitation on' the scope of the inve'ntion.

What I claim is:

1. Apparatus for treating fusilble material comprising a crucible containing said material to be treated, a heating current source'connected by a first conductor to said crucible, an electrode immersed in said material and disposed in direct electrical contact with said material and connected by a second conductor to said source, said material completing the electrical circuit between said source, said crucible and said electrode, the heat generated in said material by the conduction of said current through said material causing the melting of said material surrounding said electrode, means for circulating a fluid for coolin through said crucible and said electrode, said crucible and said electrode being constructed of a metal having thermal and electrical conductivity equal to at least that of copper.

2. Apparatus according to claim 1in which said crucible has an elongated storm and in which there is provided means for moving said electrode in a-direction parallel tothe largest dimension of said crucible.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES 'Ptf annz Z one Melting, April 1958, pages 74 to 82. Lawson: Prep. of Single Crystals, 1958, London, ButterWorths Scient. PubL, pages 62 and 6.3. a 

1. APPARATUS FOR TREATING FUSIBLE MATERIAL COMPRISING A CRUCIBLE CONTAINING SAID MATERIAL TO BE TREATED, A HEATING CURRENT SOURCE CONNECTED BY A FIRST CONDUCTOR TO SAID CRUCIBLE, AN ELECTRODE IMMERSED IN SAID MATERIAL AND DISPOSED IN DIRECT ELECTRICAL CONTACT WITH SAID MATERIAL AND CONNECTED BY A SECOND CONDUCTOR TO SAID SOURCE, SAID MATERIAL COMPLETING THE ELECTRICAL CIRCUIT BETWEEN SAID SOURCE, SAID CRUCIBLE AND SAID ELECTRODE, THE HEAT GENERATED IN SAID MATERIAL BY THE CONDUCTION OF SAID CURRENT THROUGH SAID MATERIAL CAUSING THE MELTING OF SAID MATERIAL SURROUNDING SAID ELECTRODE, MEANS FOR CIRCULATING A FLUID FOR COOLING THROUGH SAID CRUCIBLE AND SAID ELECTRODE, SAID CRUCIBLE AND SAID ELECTRODE BEING CONSTRUCTED OF A METAL HAVING THERMAL AND ELECTRICAL CONDUCTIVITY EQUAL TO AT LEAST THAT OF COOPER. 